Support subtypes and subtype polymorphism #4195
Description
We would like to be able to define a taxonomy in the type system such that one type can be a child type of another. As a result, if you were to filter on the parent type, all nodes of the child type would be matched. For example, the types might look like this, using "inherits" as a placeholder for subtype syntax:
type Vehicle {
top_speed
name
...
}
type Car inherits Vehicle {
registration
...
}
type Aeroplane inherits Vehicle {
capacity
}
With the following mutations:
_:car1 <dgraph.type> "Car" .
_:car1 <registration> "AAAA" .
_:car1 <name> "Astra" .
_:car1 <top_speed> 130 .
_:plane1 <dgraph.type> "Aeroplane" .
_:plane1 <capacity> 200 .
_:plane1 <name> "747" .
_:plane1 <top_speed> 360 .
the following query:
{
all_vehicles(func: type(Vehicle)) {
dgraph.type
name
top_speed
}
}
should return every node that is a subtype of Vehicle, i.e.:
{
"all_vehicles": [
{
"dgraph.type": "Car"
"name": "Astra"
"top_speed": 130
},
{
"dgraph.type": "Aeroplane"
"name": "747"
"top_speed": 360
}
]
}
As a workaround, right now we can simply assign all of the parent types of a particular type to an object, since it's possible to assign multiple types to a node. For example, the following mutation could be made for the car object above:
_:car1 <dgraph.type> "Car" .
_:car1 <dgraph.type> "Vehicle" .
The main issue with this approach is that when returning the type, as I did in the "all_vehicles" query above, it's not clear which type should be returned as there's no information about which is the parent and which is the child type. If a node has several types in a taxonomy, we want to return only the leaf node type.